Cell: Structure & Function

CELL: STRUCTURE AND FUNCTION
PRADEEP SINGH, SHALU SINGH
M.Sc. Medical Biochemistry
HIMSR, Jamia Hamdard

INTRODUCTION
 Fundamental unit of life.
 Self replicating structure
 The first cells were observed
and named by Robert Hooke in
1665 from slice of cork.
 Some organisms consist of a single cells =
unicellular organism, others are multicellular.
 Diameter- 2-120µm.

CELL THEORY
 Proposed by Matthais Schleiden and Theodor
Schwann in 1839:-
-All living things are made up of cells.
-Cells are the smallest working unit of all living
things.
-All cells come from pre-existing
cells through cell division.

CONSTITUENTS
 Different substances that make a cell are collectively called
Protoplasm.
 Protoplasm is composed of :-
1)Water - 70-80% Water is present in cell.
2)Carbohydrates
3)Lipids
4)Proteins
5)Electrolyte - Sodium (Na+), Potassium (K+), Magnesium (Mg2+),
Calcium (Ca2+), Phosphate , Chloride (Cl-), and Bicarbonate (HC03 - ).

SUBCELLULAR STRUCTURES
Major Structures Present in a cell are :-
Cell Membrane
Cytoplasm and its Organelles
Nucleus

CELL MEMBRANE
 Thin pliable elastic outermost
structure which envelops the
cell.
 It consists of bilipid layer with
embedded proteins that are-
i)Integral Proteins
ii)Peripheral proteins

FUNCTIONS OF CELL MEMBRANE
 Protective:- Forms outermost boundary of the cell organelles.
 Digestive:- Takes in food and excretes waste products.
 Selective Permeability:- a)Non-Polar Molecules- Gases (like O2 ,CO2, N2 ),
Lipids,Steroid Hormones, Alcohols can dissolve in the non –polar regions of
the membrane and move rapidly across the membrane.
b)Polar molecules:- H2O soluble ions , Glucose, urea etc. have much lower
solubility . Therefore Penetrate the membrane much more slowly.
 b)Chemical and Physical Properties of membrane control the free passage
of ions in and out of cell. This property helps in maintaining components in
ICF and ECF.

 Links adjacent cells together by junctional complexes to form
tissues.
 Insulating Properties:- It acts as dielectric material of a charged
condenser, thus cell membrane have very high insulating value

CYTOPLASM
 Thick, gel-like semitransparent fluid that is found in both plant and animal
cell.
 The constituent parts of cytoplasm are
cytosol, cell organelles and cytoplasmic
inclusions.
 Bounded by the plasma membrane, and
contains many organelles in a eukaryotic
cell (cell containing
membrane bounded nucleus).

CYTOSOL
 The cytosol, the aqueous part of the cytoplasm outside all of the organelles,
also contains its own distinctive proteins.
 It accounts for almost 70% of the total cell volume.
 Gelatinous substance consisting mainly of cytoskeleton filaments, organic
molecules, salt and water.
 Chemically, the cytoplasmic matrix is composed of many chemical elements
in the form of atoms, ions and molecules.

ORGANELLES
 Following organelles are present in the Cytoplasm:-
i)Mitochondria ii)Endoplasmic Reticulum
iii)Lysosomes iv)Golgi Appartus
v)Peroxisomes vi)Vacuole
 Each organelle is bounded by a lipid membrane, and has specific
functions.

MITOCHONDRIA
 The mitochondria were first observed by Kolliker in 1850 as granular
structures in the striated muscles.
 Mitochondria are called the 'powerhouse of the cell'.
STRUCTURE-
 Length- 5-12µm
Diameter- 0.5-1µm
 Filamentous or globular in shape.

 Components of Mitochondria are -
i)Outer Membrane
ii)Inner Membrane
iii)Intermediate Space- space between outer and inner
membranes
iv)Cristae-Infoldings of inner membrane
v)Matrix- The space enclosed by inner membrane
 The membranes are made up of phospholipids and proteins

 Outermost Membrane-
a)It contains large numbers of integral membrane proteins called Porins.
These porins form channels that allow molecules of 5000 daltons or less to pass.
b)Studded with enzymes concerned with biological oxidation .
 Interior (Matrix) of the Mitochondria
contains enzymes concerned with ‘citric
acid cycle’ and ‘respiratory chain oxidation’.
 Major metabolic pathways involved in
oxidation of carbohydrates, lipids and amino acids and part of special
biosynthetic pathways involving urea and heme synthesis are located in inner
matrix.

 Inner Membrane :
It contains ATPase and other enzymes concerned with synthesis
and metabolism of ATP.
Contains enzymes of Electron Transport Chain.
 The ultimate purpose of these mechanisms is oxidative
phosphorylation and synthesis of ATP.
 Mitochondria has some protein synthesised by Mitochondrial
DNA.

FUNCTIONS
 Power generating units of the cells.
 Important to maintain proper concentration of calcium ions within the
various compartments of the cell.
 Energy transduction through respiration.
 Responsible for thermogenesis.

ENDOPLASMIC RETICULUM
 Network of tubular and flat vesicular structures in the cytoplasm.
 An extensive network of closed,
flattened membrane-bounded sacs
called cisternae .
 Space inside the tubules
is filled with Endoplasmic Matrix.

 TWO TYPES-
Smooth Endoplasmic Reticulum Rough Endoplasmic Reticulum
 Ribosomes absent
Site of synthesis of lipid and
steroid hormones.
Mainly present in lipid forming
cells such as adipocytes,
interestitial cells of testis,
glycogen storing cells of liver,
adrenal cortex cells, muscle cells,
leucocytes etc.
Contains ribosomes
Site of protein
synthesis,processing and
packaging.
Mainly present in protein
forming cells such as pancreatic
acinar cells ,Goblet cells
,antibody producing plasma
cells, Nissl’s granules of nerve
cells etc.

FUNCTION
 Synthesis of proteins.
 Protein segregation.
 Unsaturation of fatty acid.
 Muscle contraction.ER is commomly known as Sarcoplasmic
Reticulum in muscle fibers.

GOLGI BODIES
 Golgi Bodies is a collection of membrane enclosed sacs composed of four or
more stacked layers of thin, flat enclosed vessels lying near the side of the
nucleus.
 Consist of multiple discrete
compartments.
 Consist of four functionally
distinct regions:
i)The cis Golgi network
ii)Golgi stack –which is divided into
- a) The medial and
- b) Trans sub compartments
iii)The trans Golgi network.

FUNCTION
 Wrapping and Packaging department of the cell.
 Produces secretion granules i.e. membrane enclosed complexes, which store
hormones and enzymes in the protein secreting cells, it packages proteins.
 Site of formation of lysosomes i.e. large irregular structures surrounded by
membrane which are present in the cytoplasm.
 It adds certain carbohydrates to form glycoproteins, which play an important
role in the association of the cells to form tissues

LYSOSOMES
 Diameter- 250 -750nm
 These are the irregular structures
surrounded by the unit membrane.
 More acidic than rest of the
cytoplasm and external bacteria
as well as worn out cell
components
are digested in them.
 The interior is kept acidic(near pH
5.0) by the action of proton pump or H+
or ATPase.
 Lysosomes are cell hydrolases and they function best at the acidic pH.

FUNCTIONS
 Acts as a form of digestive (lytic ) system or the cell, because enzymes
present in it can digest essentially all macromolecules.
 Engulf worn out components of the cells in which they are located.
 Engulf exogenous substances e.g. bacteria and degrade them.
 When a cell dies ,lysosomal enzymes causes autolysis of the remanant .
Thats why lysosomes are called as Suicidal Bags.

PEROXISOMES
 Diameter- 0.5µm
 A lipid bilayer membrane surrounds
which regulates what enters or exits the
peroxisomes.
 Urate oxidase crystalline core.
 Structure is similar to that of the
lysosomes but with a different chemical
composition .
 Peroxisomes can be formed by the budding of ER, or by division

CONTD.
 Contains oxidases that produces H2O2.
 Catalases degrades hydrogen peroxide to yield water and oxygen
 Proteins are directed to the Peroxisomes by a unique signal
sequence with the help of protein chaperones, Peroxins.

FUNCTION
 H2O2 metabolism and detoxification
 Helps in Photorespiration in plants
 Biosynthesis of lipids .
 Cholesterol and dolichol are synthesized in animals.
 Synthesis of bile acids in liver.
 Synthesis of plasmalogens ( myelin sheath).

CYTOSKELETON
 System of fibers that not only maintains the structure of the cell but also
permit it to change shape and move.
 The cytoskeleton is made up primarily of:-
i)Microtubules
ii)Intermediate Filaments
iii)Microfilaments
along with protein that anchor and
tie them together.

 Microtubules- These are long hollow structures approx. 25nm in diameter.
Determine shape of the cell, role in the contraction of the spindle and
movement of chromosomes and centrioles as well as in ciliary and flagellar
motion.
 Intermediate Filaments- They are 8-14nm in diameter and are made up of
various subunits. They form a flexible scaffolding or cell and help it resist
external pressure.
In their absence cell ruptures more easily and when they are abnormal in
human, blistering in common.
 The proteins that makeup intermediate filament are cell types specific and
are thus frequently used as cellular markers.
 Microfilaments- They are long solid fibers 4-6 nm in diameter. They
comprise the contractile protein actin and are responsible for the cell
motion.

FUNCTION
They are involved in the:-
 Movement of the chromosomes
 Cell movement
 Processes that move secretion granules in the cell
 Movement of proteins within the cell membrane.

THE NUCLEUS
 The nucleus contains chromatin, RNAs, and nuclear proteins move freely in
aqueous solution.
 Nucleus has an internal structure
that organizes the genetic material
and localizes nuclear functions.
 A loosely organized matrix of
nuclear lamins extends from the
nuclear lamina into the interior of
the nucleus.

CONTD.
 These lamins serve as sites of chromatin attachment and
organize other proteins into functional nuclear bodies.
 Chromatin within the nucleus is organized into large loops of
DNA, and specific regions of these loops are bound to the lamin
matrix by lamin-binding proteins in the chromatin.

NUCLEAR ENVELOPE
 Complex structure consisting of two nuclear membranes, an underlying
nuclear lamina, and nuclear pore complexes.
 Two concentric membranes, called the
inner and outer nuclear membranes .
 The outer membrane is continuous with the
endoplasmic reticulum, so the space between
the inner and outer nuclear membranes is
directly connected with the lumen of the
endoplasmic reticulum.

CONTD.
 Nuclear membrane is permeable only to small nonpolar
molecules.
 Underlying the inner nuclear membrane is the nuclear lamina, a
fibrous meshwork that provides structural support to the nucleus.

FUNCTION
 Serves both as the repository of genetic information and as the cell's
control center.
 The presence of a nucleus thus allows gene expression to be regulated by
posttranscriptional mechanisms, such as alternative splicing.
 The nuclear envelope provides novel opportunities for the control of gene
expression at the level of transcription.

SUMMARY
 Plasma Membrane
 Cytosol
 Mitochondria
 Endoplasmic Reticulum
 Golgi apparutus
 Lysosomes
 Peroxisomes
 Cyotoskeleton
 Nucleus
 Transport of ions and molecules
 Metab. of carbohydrate, lipids and
amino acids
 Energy production
 Synthesis of proteins and lipids
 Modification and sorting of proteins
 Cellular digestion
 Utilisation of H2O2
 Cell Morphology and cell motility
 DNA synthesis and Repair
COMPARTMENTS MAJOR FUNCTIONS

Cell: Structure & Function

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